Gas turbine engines include a compressor, combustor and turbine in flow series along a common shaft. Compressed air from the compressor is mixed with fuel in the combustor to generate hot combustion gases that rotate the turbine blades and drive the compressor. Improvements in the thrust and efficiency of gas turbine engines are linked to increasing turbine entry temperatures, which places a heavy burden on turbine blades.
Higher temperature requirements have led to the investigation and use of more heat resistant materials, including ceramic matrix composites (CMCs) in general and silicon carbide (SiC)-based CMCs in particular. Besides high temperature capability, CMCs generally exhibit low densities and low coefficients of thermal expansion. In some instances these CMC components may be employed alongside conventional metal (or metal alloy) components. For example, a metallic spar positioned inside a CMC airfoil, such as a CMC vane, may provide mechanical support and cooling air for the airfoil.
A challenge is avoiding potentially detrimental reactions at high temperatures between metallic elements (e.g., nickel and/or cobalt) in the metal component and unreacted elements (such as silicon) in the CMC component. Unreacted silicon remaining in the CMC component after melt infiltration with silicon or a silicon alloy may be referred to as “free” silicon. At high temperatures, the free silicon present in the CMC component may become highly mobile and diffuse into the metal component, leading to the formation of low melting-temperature phases and/or causing other problems.